The guys on Haskell Cafe delight in complex problems (and solutions). There's a list for beginners, too.

You will enjoy the Haskell Cafe later.

By the way, one small exercise (from a paper by Chris Okasaki): Write a function with the type

  Tree a -> Tree Int

where

  data Tree a = Node a (Tree a) (Tree a) | Empty

your function should all nodes in a tree with their position in level order traversal (also called breadth first search). E.g. (lot == lever order traversal)

  lot Empty = Empty
  lot (Node 'a' (Node 'b' Empty (Node 'c' Empty Empty))
                (Node 'd' Empty Empty)) =
      (Node '0' (Node '1' Empty (Node '3' Empty Empty))
                (Node '2' Empty Empty))